Defective nozzle compensation

ABSTRACT

An image recording apparatus ( 2000 ) is disclosed, which comprises forming elements ( 2002 ) for forming an image, a memory ( 2006 ) indicating relative desirability of utilising the forming elements ( 2002 ) for forming the image, and processing means ( 2008 ) for computing image recording signals using input image signals ( 2012 ) and the stored data. The use of a particular forming element is thereby biased dependent upon the relative desirability data of other forming elements, the corresponding input signal for the particular forming element, and a term for the particular forming element.

COPYRIGHT NOTICE

[0001] This patent specification contains material that is subject tocopyright protection. The copyright owner has no objection to thereproduction of this patent specification or related materials fromassociated patent office files for the purposes of review, but otherwisereserves all copyright whatsoever.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates generally to printers, and inparticular to ink jet printers.

BACKGROUND ART

[0003] Digital image printing systems which use multiple ink nozzlesintegrated within a print head have increased rapidly in popularity inrecent years. Either at manufacture, or during operation, the particularprinting nozzles within a print head can fail to deliver ink droplets ofthe required size, or alternatively, with the required positionalaccuracy. These problems can arise from a combination of manufacturingflaws, and wear and tear. In extreme cases, individual nozzles can failto deliver any ink whatsoever, due to being blocked or damaged.

[0004] Defective nozzles, be they blocked or merely defective in termsof their performance, can be identified manually or automatically byexamining test print output.

[0005] Even where nozzles delivery ink droplets satisfactorily, thenozzles can vary in their characteristics, and produce uneven printdensities, U.S. Pat. No. 5,038,208 entitled “Image Forming Apparatuswith a Function for Correcting Recording Density Unevenness” describes amethod and apparatus related to improving the evenness of print densityproduced by nozzles of varying characteristics. The patent discloses aprinter having a multi-nozzle printing head, this printer also storingdata associated with the image-forming characteristics of each of themulti-nozzle heads. Furthermore, a corrector means for correcting theimage forming signals based upon the data stored in memory correspondingto the multi-nozzle head characteristics is disclosed. Theaforementioned correction is performed either by retrieving a correctedvalue from a memory, or by retrieving a correction value from a memory,and adding the correction value to the uncorrected image value. In bothcases, the image correction data is accessed using both the uncorrectedimage value, and a nozzle counter value. This type of correction isknown as “head shading”. Head shading methods do not, however,satisfactorily eliminate print artefacts where a blocked nozzle ispresent, or where the ink ejection performance of a nozzle fails to meetminimum requirements.

DISCLOSURE OF THE INVENTION

[0006] It is an object of the present invention to substantiallyovercome, or at least ameliorate, one or more disadvantages of existingarrangements.

[0007] According to a first aspect of the invention there is disclosedan image recording apparatus comprising:

[0008] (a) a plurality of forming elements for forming an image usingimage recording signals, said image according with a correspondingplurality of input image forming signals;

[0009] (b) memory means for storing data for said forming elementsindicating the relative desirability of utilising said forming elementsfor forming the image; and

[0010] (c) image processing means for computing said image recordingsignals using said input image forming signals and said data stored insaid memory means, wherein the use of a particular forming element isthereby biased dependent upon the relative desirability data of otherforming elements, the corresponding input image forming signal for theparticular forming element, and a term for the particular formingelement.

[0011] According to another aspect of the invention, there is disclosedan image recording apparatus comprising:

[0012] (a) a plurality of forming elements for forming an imageaccording to input image forming signals;

[0013] (b) memory means for storing data for said forming elementsindicating the relative desirability of utilising said forming elementsfor forming an image;

[0014] (c) image signal modification means for redistributing values ofsaid input image forming signals based on said data stored in saidmemory means so as to bias the use of said forming elements, wherein theuse of a particular one of said forming elements is thereby biaseddependent upon the relative desirability data of other forming elements,a corresponding input image forming signal for the particular formingelement, and a term for the particular forming element.

[0015] According to another aspect of the invention, there is provided amethod, in printing an image, of compensating for one or more defectiveprinter nozzles in a plurality of printer nozzles, said methodcomprising the steps of:

[0016] biasing, for each first image value associated with a firstnozzle, at least one second image value associated with another nozzle,said biasing being dependent upon said first image value and a term forsaid first nozzle; and

[0017] printing the image in accordance with the biased image values,said biasing reducing print artefacts otherwise caused by the one ormore defective nozzles.

[0018] According to another aspect of the invention, there is provided amethod of printing a multi-level halftoned image comprising the stepsof:

[0019] adjusting a relationship between input image values andcorresponding average halftone output values.

[0020] The biasing of other nozzles can be performed in various ways.This includes image value redistribution from a defective nozzle toimmediately neighbouring nozzles of a same colour. The redistributioncan be either partial, being constrained by a normal operating range ofthe neighbouring nozzles, or complete, in the case where neighbouringnozzles can support extended range, ie “super-intensity” printing.Alternatively, or in addition, cross colour compensation can beperformed, whereby biasing is performed with respect to a correspondingnozzle of a different colour. The increased range of super-intensityimage values can be compensated for by using range-reduction mapping, inparticular, based upon checkerboard quantisation. In the event thatbiased images are subsequently halftoned, a transfer function betweeninput image values and corresponding average halftone output values canbe adjusted, in order to tune utilisation of super-intensity printing.

[0021] According to another aspect of the invention, there is providedan apparatus for implementing any one of the aforementioned methods.

[0022] According to another aspect of the invention there is provided acomputer program product including a computer readable medium havingrecorded thereon a computer program for implementing any one of themethods described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] A number of preferred embodiments of the present invention willnow be described with reference to the drawings, in which:

[0024]FIG. 1 depicts a prior art halftoning arrangement;

[0025]FIG. 2 illustrates a prior art unevenness correction, andhalftoning arrangement;

[0026]FIG. 3 shows the unevenness correction process in FIG. 2 in moredetail;

[0027]FIG. 4 depicts an arrangement for defective nozzle compensation inaccordance with a preferred embodiment of the present invention;

[0028]FIG. 5 shows a full-width fixed head printer whereupon thearrangement in FIG. 4 can be applied;

[0029]FIG. 6 shows a shuttle-head printer, to which the arrangement inFIG. 4 can be applied;

[0030]FIG. 7 illustrates redistribution of image values from a defectivenozzle to neighbouring non-defective nozzles in a first embodiment ofthe invention;

[0031]FIG. 8 shows a process for redistribution of image values inaccordance with the arrangement illustrated in FIG. 7;

[0032]FIG. 9 illustrates a pattern of printed dots in a single colourwhich is output by the arrangement in accordance with FIG. 7;

[0033]FIG. 10 shows defective nozzle compensation using cross-colourcompensation according to a first embodiment of the present invention;

[0034]FIG. 11 provides an example of printable dots-per-nozzle for apixel in accordance with a second embodiment of the present invention;FIG. 12 illustrates “full” paper coverage by dots of a single colourboth with and without defective nozzles being present;

[0035]FIG. 13 shows the application of quantisation correction in apreferred embodiment of the present invention;

[0036]FIG. 14 illustrates tuning a three level halftone error diffusiontransfer function;

[0037]FIG. 15 depicts a process for printing a halftoned image using atuned error diffusion table;

[0038]FIG. 16 depicts a prior art error diffusion arrangement;

[0039]FIG. 17 shows an untuned 3 level halftone error diffusion table:

[0040]FIG. 18 shows a tuned 3 level halftone error diffusion table;

[0041]FIG. 19 depicts a general purpose computer upon which preferredembodiments of the invention can be practiced; and

[0042]FIG. 20 depicts a block diagram representation of an apparatus forredistribution of image values from a defective nozzle to neighbouringnon-defective nozzles

DETAILED DESCRIPTION INCLUDING BEST MODE

[0043] Where reference is made in any one or more of the accompanyingdrawings to steps and//or features, which have the same referencenumerals, those steps and/or features have for the purposes of thisdescription the same function(s) or operation(s), unless the contraryintention appears.

[0044] The principles of the preferred methods described herein havegeneral applicability to printing devices which have a multiplicity ofprint-forming means integrated into a single printing head. However, forease of explanation, the steps of the preferred methods are describedwith reference to ink jet printers. It is, however, not intended thatthe present invention be limited thereto.

[0045] Arrangements of the present invention are applied to imageprinting systems, having recording heads each of which has an array ofrecording elements, and which uses data describing defective recordingelements in order to provide an improved printer output. For ease ofillustration, the description is directed to multiple print heads eachhaving a linear array of ink jet nozzles, however the embodiments can beextended to other types of printing systems.

[0046] A common aspect of the apparatus and methods encapsulated in thedescribed embodiments is the use of defective nozzle data. This is usedfor reducing the use of defective nozzles, and for correspondinglyincreasing use of non-defective nozzles which print in the vicinity ofthe positions at which the defective nozzles print. This has the effectof reducing print artefacts caused by the defective nozzles. Whereinter-nozzle spacing is sufficiently small, the aforementionedredistribution of nozzle use, or “cross-nozzle” compensation, results insignificant reduction in artefacts. Accordingly, print heads which areeither manufactured with defective nozzles, or which develop defectivenozzles over time, can continue to be used while producing good qualityoutput.

[0047] One approach for providing cross-nozzle compensation is to reduceimage values associated with defective nozzles, correspondinglyincreasing the image values associated with neighbouring, non-defectivenozzles. This can be achieved by a image redistribution process which isfurther described in relation to the first and second embodiments (egsee FIGS. 4 and 9). In colour printing, an additional approach forproviding cross-nozzle compensation is available. This is to compensatefor a defective nozzle of a first colour component by increasing animage value associated with a corresponding nozzle of a second colour,ie one which prints at the same position as the defective nozzle. Thisis described further with respect to the first embodiment (eg see FIG.9).

[0048] In a first arrangement, image value redistribution is performedin a manner which restricts the redistribution so that resultant imagevalues of the receiving nozzles (ie., the neighbouring nozzles), remainwithin their “normal” range. The “normal” range of a printer nozzle isconsidered to be that range which, during normal operation, supports theprinting of intensity values between “0” and “maximum”, or in 8-bitnumeral terms, between 0 and 255. In the first embodiment, since thereis a restriction on the amount of error which can be redistributed toneighbouring nozzles, in general the complete image value assigned to adefective nozzle will not always be able to be completely redistributedThis can be overcome, by using the residual, ie undistributed, imagevalue to perform compensation by manipulating the image values of othercolour components.

[0049] In a second arrangement, image value redistribution is performedin a manner which allows the receiving nozzles to exceed their normalrange, and an increased, (“superintensity”) ink ejection per colourcomponent per pixel is permitted, beyond that which is required toprovide normal full ink coverage of the paper. The second embodimentalso discloses a method of tuning how often the superintensity inkejection is provided.

[0050] in an ink-jet printer, image data is typically converted tonozzle firing control data. The image data is typically multi-tonecolour data which must be converted into nozzle firing data in order tocontrol the ink ejection of the nozzles of the print heads.

[0051] The nozzle firing data for a multi-tone colour image can beconsidered as a colour image with a reduced number of colour intensityvalues per pixel. The process of determining image data with a reducednumber of intensity values per pixel is referred to as “halftoning”.

[0052] For ease of explanation, a rectangular array of pixels isassumed, with each pixel having an intensity value for each one of thecolour components Cyan, Magenta, Yellow and Black. Equivalently, themulti-tone colour image can be considered as a set of four rectangulararrays of intensity values, one array per colour, each array having thesame physical dimensions. Intensity values are represented as 8-bitnumbers, having a value in the range of 0 to 255. Value 0 corresponds toa minimum colour density (i.e., no deposition), and the value 255corresponds to a maximum colour density (i.e., full ink deposition).

[0053] Nozzle firing data is generated from the aforementioned imagedata, and comprises an array of nozzle firing values, one array for eachcolour component.

[0054] Again for ease of explanation, for each colour component, thedimensions of the nozzle firing array are assumed to be the same as thedimensions of the corresponding array of multi-tone image intensityvalues. Accordingly, it is assumed that there is a one-to-onecorrespondence between multi-tone image pixels, and positions in anozzle firing array, so that the description is not burdened by spatialresolution conversion issues. In the event that nozzle separation in aprint head is different to the input image pixel spacing, image valuesfor nozzles can typically be derived from pixel image values byinterpolation, or by resolution conversion algorithms. After suchconversion, defective nozzle compensation can then be applied to thenozzle image values.

[0055] Typically, for each colour component, there is one nozzle firingposition for each position in a nozzle firing array. This is not alwaysthe case however. For example, in the second embodiment, there are twonozzle firing positions corresponding to each position in a nozzlefiring array.

[0056] In the first arrangement, each nozzle firing value is a 1-bitnumber, where the value “0” indicates that ink should not be ejected forthat position in the nozzle firing array, and a value “1” indicates thatink should be ejected. In the second embodiment, each nozzle firingvalue may be 0, 1 or 2, this value indicating the number of ink dropletsto be ejected for that position in the nozzle firing array.

[0057]FIG. 1 depicts a prior art halftoning arrangement. A multi-tone 8bit image value 100 is input into a halftoning process 102, whichproduces a nozzle firing value 104. The halftoning process is oftendescribed as a “binarisation” process, since in the present case itgenerates data describing whether a nozzle should fire or not at eachnozzle firing position. Halftoning may be performed by error diffusionor dithering, and typically, each colour component is halftonedindependently.

[0058] In FIG. 2 the 8-bit multi-tone image array value 100 is processedby an unevenness correction process 200 prior to the halftoning process102. This optional prior art arrangement is described in the cited U.S.Pat. No. 5,038,208.

[0059]FIG. 3 provides more detail about the aforementioned unevennesscorrection process. Each nozzle is associated with a curve whichcharacterises its performance. This curve is a mapping, for each nozzle,from an uncorrected 8-bit image value, to an unevenness-corrected 8-bitimage value. A nozzle counter value 300 is used to retrieve a curvevalue from a nozzle curve table 302. The curve value 304 which isselected and output is processed in accordance with the 8 bit multi-toneimage array value 100 in a curve table process 306. This process 306produces an unevenness corrected image value 202.

[0060]FIG. 4 illustrates a defective nozzle compensation arrangement.The 8 bit multi-tone image array value 100 is input into a defectivenozzle compensation process 400. This process 400 outputs aredistributed image value 402. This redistributed value 402 has the sameprecision, i.e., 8-bits, as the multi-tone image array value 100.Thereafter, the redistributed image value 402 is input into anunevenness correction process 200, and subsequently, into the halftoningprocess 102, in order to produce the final processed nozzle firing arrayvalue 104. It is noted that the defective nozzle compensation process400 is performed prior to the unevenness correction process 200, andprior to the halftoning process 102. Alternatively, defective nozzlecompensation 400 can be performed prior to the halftoning process 102,and the unevenness correction process 200 can be omitted.

[0061]FIG. 5 illustrates a full-width fixed head printer, in which asheet of paper 500 having a width 502 and being fed in a directiondepicted by an arrow 504 passes beneath a full width print head 506. Asegment 514 of the stationary print head 506 is shown in more detail inan inset 508, where it is seen to comprise a plurality of individualprint nozzles 510. A print head for each colour component is provided,each of these heads being oriented at right angles to the paper feeddirection depicted by the arrow 504. The print head 506 is fixed, andink is ejected from each of the print head nozzles, e.g., 510, while thepaper is advanced underneath the print head. An entire row (i.e.,scanline) of the image is printed at a time. An individual printernozzle, e.g., 510, is restricted to printing dots of a column of pixels,since the print head 506 is fixed in position.

[0062]FIG. 6 illustrates a shuttle-head ink jet print system, in which asheet of paper 600, having a width 602, is fed in a direction depictedby an arrow 604. A shuttle print head 612 moves at right angles acrossthe paper 600, as depicted by a bi-directional arrow 610. A segment 620of the print head 612 is shown in more detail 606 in an inset 618. Thisshows that the print head 612 comprises a plurality of individualprinter nozzles 608. A print head 612 is provided for each colourcomponent, and each print head 612 is oriented parallel to the paperfeed direction as depicted by the arrow 604. The print heads 612 moveover the paper at right angles 612 to the paper feed direction 604, withink being ejected from the print head nozzles 608 while the paper 600 isstationary, and the print heads 612 are performing a shuttle scan acrossthe page 600. A band of scanlines is printed in one pass of the shuttleprint heads. The width of the band of scanlines printed in a single passof the shuttle print head 612 is restricted by the length of the printhead 614. In any given pass of the shuttle print heads, an individualnozzle, 608 is restricted to printing dots of a particular scanline(i.e., a row) of pixels. Between passes of the shuttle print head 612,the paper 600 is advanced in the paper feed direction 604, so thatsuccessive bands of scanlines can be printed. Typically, to reduceprinter artefacts due to nozzle inconsistencies, the paper 600 isadvanced in a direction 604 by only a fraction of the length 614 of theband of scanlines printed by one pass of the shuttle print head. In thismanner, each scanline is printed using multiple passes of the shuttleprint head 606. Dots of a colour component of a scanline are accordinglyprinted by a different nozzle of the same print head for each pass.

[0063] In both styles of printer, i.e., the full-width fixed headprinter (see FIG. 5), and the shuttle-head printer (see FIG. 6), therelative movement between the print heads (506, 612) and the paper (500,600) is at right angles to the respective print head. Each nozzle (510,608) prints a line of dots. Neighbouring nozzles print neighbouringlines of dots.

[0064]FIG. 7 illustrates image value distribution from a defectivenozzle to neighbouring non-defective nozzles. A line of nozzles 700 to702 within a print head (not shown) is depicted. A defective nozzle 704is indicated by an “X” i.e., 706. A line of rectangular pixels 708 to710 is shown adjacent to the line of nozzles 700 to 702. Theaforementioned representation is equally applicable to a full-widthfixed head printer, and to a shuttle head printer arrangement. A graphwith axes of image value 712 against pixel number 714 illustrates adesired sequence of descending image values 716 to 724. These are theassigned image values which are desired to be printed by the print head.Since the nozzle 704 is defective, the desired image value 720 cannot beprinted. The graph of actual image value 726 against pixel number 728illustrates the actual printed image values after image valueredistribution. It is noted that the image value 730 which correspondsto the defective nozzle 704 has a 0 image value, while the immediatelyneighbouring pixels have image values 732 and 734, these having beingincreased in order to compensate for the 0 image value 730. The resultof redistributing the image value from the defective nozzle 704 to theneighbouring nozzles 736 and 738 is that dots which would otherwise havebeen allocated to the defective nozzle, were it not defective, areinstead printed by the nozzles 736, 738 which are situated on eitherside of the defective nozzle 704.

[0065]FIG. 8 shows a process for redistribution of image values inaccordance with the arrangement described in regard to FIG. 7. In afirst step 1802 of the process 1800, data corresponding to relativedesirability of using various forming elements are determined andstored. Thereafter, in a step 1804, an input image signal for a currentnozzle is input. In a following decision step 1806, a determination inmade, depending for example, upon a measure of desirability for thecurrent nozzle, whether bias is required. If bias is indeed needed, thenthe process 1800 is directed in accordance with a “yes” arrow to a step1808 in which an input image signal for another nozzle is input. In afollowing step 1810, some or all of the input image signal for currentnozzle is distributed to the other nozzle, ie. the input image signalfor the current nozzle is added to the input image signal for the othernozzle. Thereafter, in a step 1812, the current nozzle is fired, therebydistributing ink in accordance with redistributed signal upon theprinting medium. In a following step 1814, an index for the currentnozzle is incremented, after which the process 1800 is returned to thestep 1804. Returning to the decision step 1806, if the decision stepdetermines that bias is not after all required, then the process 1800 isdirected in accordance with a “no” arrow to the step 1812, which firesthe current nozzle. It is noted that the initial step 1802, in whichrelative desirability data for the various forming elements are stored,is performed only at the outset of the process 1800.

[0066]FIG. 9 depicts a patterns of dots of a single colour componentwhich are to be printed according to values in a nozzle firing array.Dots can be printed at positions on a rectangular grid 800, whichconsists of vertical grid lines 804 and horizontal grid lines 802. Inthe figure, dots have been shown in outline only, eg. 806, so that thepixel grid 800, remains visible. The rectangular grid 800 allows for onedot per grid position. All dots of a particular grid column are printedby the same nozzle. In the FIGURE, a nozzle corresponding to the centrecolumn of pixels 812 is defective. The illustrated dot outlinescorrespond to a nozzle firing value array generated using defectivenozzle compensation by image value distribution, followed by halftoning,for an image region of near 50% intensity. As a result of the defectivenozzle compensation by image value distribution, no dots are printed ingrid column 812, and accordingly, additional dots 808, 810 and 811 areprinted in the neighbouring grid columns. Due to the additional dots808, 810 and 811, average ink deposition near the blocked nozzle is notreduced, and accordingly, the 50% intensity of the image values isreproduced in the average ink deposition. Advantageously, this desiredaverage ink deposition is maintained with reduced print artefacts due tothe defective nozzle. This advantageous performance is maintained bothwhere the defective nozzle is blocked, or alternatively, where it isdefective and, for example, ejects ink unreliably, too far to one sideor the like.

[0067] A C programming language code fragment is now presented inrelation to a first embodiment of the present invention, i.e., defectivenozzle compensation by restricted image value redistribution for afull-width fixed head printer. For ease of explanation, it is assumedthat (i) there is one nozzle per pixel of a scanline, and (ii) thedefective nozzle data consists of a one-bit value for each nozzle,indicating whether or not the associated nozzle is defective, and (iii)image intensity is redistributed using only nearest neighbour nozzles,i.e., pixels. clamp_val = 255; .. /* The following processing isperformed one pixel at a time (ie one nozzle at a time) from the firstpixel of a scanline to the last. */ /* if the current pixel's nozzle isdefective and the next pixel's nozzle is not defective then distributeas much as possible of remaining intensity of current pixel to nextpixel */ if (defective[n] && !defective[n + 1]) { old_val = image[n +1]; new_val = image[n + 1] + image[n]; if (new_val > clamp_val) {new_val = clamp_val; } image[n + 1] = new_val; image[n] −= new_val −old_val; } /* if the current pixel's nozzle is not defective and thenext pixel's nozzle is defective then distribute as much as possible ofhalf intensity of next pixel to current pixel */ if (!defective[n] &&defective[n + 1]) { old_val = image[n]; new_val = image[n] + image[n +1]/2; if (new_val > clamp_val) { new_val = clamp_val; } image[n] =new_val; image[n + 1] −= new_val − old_val; }

[0068] Prior to image value re-distribution, the 8 bit image values fallinto the range 0 to 255, and accordingly, image values of 0 and 255represent an unbiased operating range of 0% and 100% respectively. Theabove code associated with the first embodiment restricts the dynamicrange after re-distribution to the same range ie 0 to 255, andaccordingly, image values of 0 and 255 represent a biased operatingrange of 0% and 100% respectively.

[0069] It is found, for the case where nozzle separation is equal to{fraction (1/600)} of an inch, that restricted image valueredistribution as described above significantly reduces streakingartefacts which typically occur in the presence of a blocked ornon-firing nozzle.

[0070] When the image region to be printed consists of a constant imagevalue which is less than or equal to (⅔) *255, all of the image value ofthe non-firing nozzle is, by virtue of the aforementioned image value,able to be distributed to immediately neighbouring nozzles. For suchimage regions, defective nozzle compensation using restricted imagevalue redistribution as described previously, provides a clear reductionin the streaking artefact which typically occurs as a consequence of anon-firing nozzle.

[0071] In contrast however, where the image region to be printedconsists of a constant image value greater than ({fraction (213)}) *255,then some of the image value of the non-firing nozzle is unable to bedistributed to its immediately neighbouring nozzles. In such cases, astreak due to the non-firing nozzle remains visible even after defectivenozzle compensation using the restricted image value redistribution asdescribed.

[0072] In summary, defective nozzle compensation using restricted imagevalue redistribution is limited by the range restriction of theimmediately neighbouring print nozzles. This means that a residual imagevalue, equal to the amount which could not be redistributed, is“retained” by the blocked nozzle.

[0073] The concept of image redistribution can, however, be extended aswill now be described. In a multi-colour printing arrangement, theaforementioned residual image value, ie. the value which cannot beredistributed to immediately neighbouring nozzles, can nonetheless beused for “cross-colour” compensation. Cross-colour compensation relatesto the fact that print artefacts which may normally be produced as aresult of a blocked nozzle of a first colour component, can often bereduced by printing dots of another colour component, in the area wheredots of the first colour component are missing In other words,cross-colour compensation is the use of other colour components toreduce artefacts due to defective nozzles in a first colour.

[0074]FIG. 10 shows how defective nozzle compensation using restrictedimage value redistribution as previously described can be extended toincorporate cross-colour compensation for the Cyan and Magenta colourcomponents where either, or both the print heads associated with Cyanand Magenta may have blocked nozzles. FIG. 10 shows an 8-bit Cyan imagearray value 900 being input into a restricted image value redistributionprocess 902. Similarly, an 8-bit Magenta image array value 910 is inputinto a restricted image value redistribution process 912. Thereafter,the 8-bit redistributed image values 904 and 914 respectively are fedinto a cross-colour compensation process 906. The cross-colourcompensation process 906 makes use of cross-colour compensation tocorrect for artefacts caused within each of the Cyan and Magenta images,resulting from respective blocked nozzles in each separate print head.The cross-colour compensation process 906 outputs an 8-bit Cyan nozzlecompensated image value 908, and an 8-bit Magenta nozzle compensatedimage value 916.

[0075] The processing performed in the cross-colour compensation process906, this being performed on a per pixel basis for both Cyan andMagenta, is described in the following C language code fragment: /* ifCyan nozzle is defective and Magenta nozzle is not defective thenaugment the Magenta image value */ if (cyan_defective[n] &&!magenta_defective[n]) { new_val = magenta[n] + f1 * cyan[n]; if(new_val > 255) { new_val = 255; } magenta[n] = new_val; } /* if Cyannozzle is not defective and Magenta nozzle is defective then augment theCyan image value */ if (!cyan_defective[n] && magenta_defective[n]) {new_val = cyan[n] + f2 * magenta[n]; if (new_val > 255) { new_val = 255;} cyan[n] = new_val; }

[0076] The aforementioned C code provides Magenta colour compensationfor a defective Cyan nozzle, Or vice versa, on a per pixel basis. Thevalues of the parameters f1 and f2 are determined by experiment. For 600dots per inch (dpi) printing, values f1 and f2 equal to 0.2 provide goodreduction of streaking artefacts due both to (i) a non-firing Cyannozzle in an image region of high Cyan and low Magenta values, and (ii)a non-firing Magenta nozzle in an image region of low Cyan and highMagenta values.

[0077] Since the Cyan/Magenta cross-colour compensation for a non-firingCyan nozzle is effective in high Cyan image regions, and sincerestricted image value distribution is effective for image regions up to⅔ maximum image value, it is seen that these two methods compliment eachother. Accordingly, for 600 dpi printing, the combination of restrictedimage value redistribution, followed by Cyan/Magenta cross-colourcompensation, provides a good reduction in streaking artefacts due toeither a blocked Cyan nozzle or a blocked Magenta nozzle, over most ofthe printer gamut.

[0078] A second arrangement is now proposed to improve the effectivenessof defective nozzle compensation by image value redistribution, thistime through the use of print systems which can eject more ink pernozzle than is typically required to achieve a full ink coverage of thepaper. In other words, this embodiment relates to extending the range ofa print nozzle beyond the 0-255 normal boundaries. This type of printingwill hereafter be referred to as super output intensity printing.

[0079]FIG. 11 shows an example of dots printable per nozzle per pixelfor a single colour component for super output intensity printing. Apixel grid 944 comprising adjacent rectangular pixel positions 946typically provides for pixel separation of {fraction (1/1600)} inch inboth directions 948. Super output intensity printing, however, enablesdots to be printed with half the aforementioned separation, ie.,{fraction (1/1200)} inch, as shown by dots 950, 952, which are separatedby {fraction (1/1200)} inch as indicated by arrows 940, 942. In thepresent example, print nozzles are separated by {fraction (1/600)} inchalong the print head, however each nozzle can fire twice during relativemovement of the head and the paper, through {fraction (1/600)} inch.

[0080] Nozzle firing data associated with pixels to be printed can berepresented as an array for each colour component, the array comprisingvalues in the range 0 to 2. Each value thus represents nozzle firingdata for a particular colour component, and for a particular pixel. Thevalue (ie. halftone output level) “0” corresponds to no dots printed forthe pixel; value “1” corresponds to 1 dot printed for the pixel, and thevalue “2” corresponds to 2 dots printed for the pixel. Since the printsystem is designed to achieve full ink coverage of the paper bydepositing 1 dot per pixel position 946, and the print system is able toeject two dots per pixel position, it is possible to ensure that theaverage volume of ink deposited in the vicinity of a nozzle is notreduced when a single nozzle is not firing.

[0081]FIG. 12 shows a pixel grid 1104 fully covered, in the idealsituation, by individual dots 1102. In the event that a print nozzle isblocked, and thus unable to fire in a column depicted by an arrow 1108,super intensity printing is used to produce additional printed pixeldots 1110, thus maintaining the required average ink deposition in theregion of the unprintable column 1108. The actual printed dot pattern1106 is an approximation to the desired printed dot pattern 1100. Superintensity output printing also takes advantage of the fact that printeddots typically swell when they overlap, and consequently, in superintensity output printing 1106, increased dot swelling can be expectedin the region of the super intensity output dots 1110, thereby reducingthe area of paper uncovered by ink 1108.

[0082]FIG. 12 describes a particular method of performing super outputintensity printing, this being by ejecting an extra ink droplet perpixel per colour component. This is depicted by overlapping dots 950,952 having a spacing half the normal spacing 940, 942 (see FIG. 11).

[0083] Alternatively, more than one extra droplet per pixel position maybe ejected, larger ink droplets may be ejected, or a combination of theaforementioned methods may be used.

[0084] The use of super intensity output printing results in full, orcomplete image value redistribution. This differs from restricted imagevalue redistribution, in that the redistribution is not limited by thenormal range of the pixels receiving the redistributed image values Theimage values of the receiving pixels can accordingly be increased beyondtheir normal maximum boundary values. Assuming that (i) there is onenozzle per pixel of a scanline, (ii) defective nozzle firing dataconsists of a 1 bit value for each nozzle, indicating whether or not theassociated nozzle is defective, and (iii) image intensity isredistributed using only nearest neighbour pixels, then the C coderelating to restricted image value redistribution which has beenprovided is applicable, except that the value used to clamp resultingimage values is changed to 510 (ie., twice the normal level of 255, andaccordingly, image values of 0 and 510 represent a biased operatingrange of 0% and 200% respectively). Given this new clamping value, andwhere there is a single defective nozzle, the total image valueassociated with the defective nozzle can be redistributed to pixelscorresponding to neighbouring nozzles. The image values of suchneighbouring pixels will now fall in a range of 0 to 510, which requires9 bits for representation. Accordingly, the original 8 bit input imagevalue, eg. 100 in FIG. 1, has now been increased to a 9 bit image valueafter full image value redistribution. It is desirable to map theresultant 9 bit image values back to the original 8 bit dynamic range,in order to avoid modifying one or more of the unevenness correctionprocess 200, and the halftoning process 102 (see FIG. 4). In particular,if the number of bits required to represent image values input to theunevenness correction process 200 is increased from 8 to 9 bits, thenthat process 200 requires either a two-old increase in the size of thecurve tables, or alternatively, more complex processing.

[0085]FIG. 13 shows an 8 bit multi-tone image array value 1200 beinginput into a full image value redistribution process 1202, which outputsa fully redistributed image value array 1204 having a 9 bit range. Thisoutput 1204 is input to a checkerboard quantisation process 1206, whichreduces the 9 bit range of image values 1204 back to an 8 bit fullyredistributed and quantised image value 1208 having a range of 8 bits.The checkerboard quantisation process 1206 effectively divides the imagevalue 1204 by 2, alternately rounding fractional image values up, ordown. The rounding is performed alternately according to a checker boardpattern. Accordingly, if the sum of (i) the pixel scanline number, axed(ii) the pixel position within the scanline, is even, then the roundingis opposite to the rounding performed when the sum is odd. Thecheckerboard quantisation process also includes at least one instance ofspecial case processing, this occurring if the input image value is 255,in which case the output image value is always rounded in the samedirection. This special case processing is provided because imageregions having an input value of 255, ie. the maximum input image value,should typically result in a halftone output corresponding to full inkcoverage of paper. If this special case processing is not included, thenthe resultant halftone output for such image regions is a mix ofhalftone output levels, this non-uniform pattern being undesirable forimage regions having a constant maximum input image value.

[0086] The checkerboard quantisation process 1206, which rescales therange of the image value 1204 from 9 bits back to 8 bits (1208),provides a number of advantages over simple truncation of image valuesto the most significant 8 bits. Although both checkerboard quantisationand simple truncation result in a range of 0 to 255 for a pixel,checkerboard quantisation produces a greater number of differentresulting local average image values for pixels of a region, given animage region of constant input image value. For checkerboardquantisation therefore, a region of an input image which is a gradualblend from one image value to another, is represented by a smoothertransition in local average image values. This reduces colour stepartefacts which typically occur for simple truncation.

[0087] Super intensity output printing, which as described requiresdeposition of additional ink, can cause problems including unwanted inkswelling, increased paper wetting, and increased ink drying time. Inorder to optimise, or maximise the beneficial effect of super intensityink deposition in the vicinity of a blocked nozzle, it is sometimesdesirable that the frequency of super intensity ink deposition per pixelbe reduced by tuning it down.

[0088] In contrast, another problem which can be associated with the useof super intensity output printing, is that if sufficient superintensity ink is not deposited on the paper often enough, then theunprinted line resulting from a defective printing nozzle will beinsufficiently compensated for. It is thus advantageous, in this case,to tune the frequency of super intensity ink deposition per pixel,typically increasing the frequency in this instance.

[0089]FIG. 14 illustrates the aspect of tuning in the context of a threelevel halftone error diffusion process. The halftoning process isrepresented by a graph having an ordinate 1400 representing the averagenumber of output dots printed (ie. 0, 1 or 2 dots per particular pixel),and an abscissa 1402 representing the input image value, ie. from 0 to255 for super intensity output printing after checkerboard quantisation.The relationship, or transfer function, between the input image valueand the average three level halftone output is typically depicted by theline 1404 which is linear across the input image value range. An inputimage value 1416 is seen to lie between the abscissa value “128” and theabscissa value “255”, and the input image value 1416 will thus, by athree level error diffusion process, map either to a halftone outputvalue of “1”, or to a halftone output value of “2”. For an input imageregion of constant input image value 1416, the proportion of cases inwhich the input image value 1416 maps to “2”, equals the differencebetween the average halftone output of the input image value 1416, andthe halftone output value “1” (ie. 1436). Turning to the lower graph inthe FIGURE, it is seen that the line 1404 has now been tuned, ie.adjusted, to take the form of three linear line segments 1410, 1412 and1414. The input image value 1420 is the same as the input image value1416, however, according to the tuned transfer function, for an inputimage region of constant input image value 1420, the proportion of casesthat the input image value maps to “2”, being the difference between theaverage halftone output of the input image value 1420, and the halftoneoutput value “1”, (ie. 1430), has increased. The effect of tuning thetransfer function, ie. the line 1404, to the set of linear line segments1410, 1412 and 1414, is thus to increase, on average, the utilisation ofsuperintensity output double pixels. The line segment 1414 ishorizontal, clamping all input image values lying between “191”, and“255” on the abscissa 1408, to an output halftoned value of “2”. Thespecific example of tuning shown in FIG. 14 is representative only, andthe feature of tuning a multi-level halftoned output is clearly notlimited thereto.

[0090] One method of tuning the halftoning transfer function andadjusting how often superintensity ink deposition is performed, isprovided by simply remapping image value prior to halftoning, forexample, by use of a lookup table. An alternative effective method oftuning the halftoning transfer function, and thereby adjusting how oftensuperintensity ink deposition is performed, is provided by performinghalftoning by error diffusion with a modified error diffusion table.This method of tuning by use of a modified error diffusion table, is nowdescribed.

[0091]FIG. 15 shows a process 1900 whereby a tuned error diffusion tableis used to print a multi-level halftoned image. In a first step 1902, atuned error diffusion table is prepared, for example using thetechniques described in relation to FIGS. 17 and 18. Thereafter, in astep 1904, an input image signal for halftoning is input into theprocess 1900. In a following step 1906, a corresponding halftone outputfor the input image signal is determined from the table. It is notedthat this will be a “tuned” output, resulting from the aforementionedstep 1902. In a following step 1908, the process 1900 determines whethermore pixels are required to be processed. If there are further pixels inthis category, then the process 1900 is directed in accordance with a“yes” arrow back to the step 1904. If, on the other hand, no more pixelsare required to be processed, then the process 1900 is directed inaccordance with a “no” arrow to a step 1910, at which stage the process1900 is terminated.

[0092]FIG. 16 shows, by way of background, example error diffusioncoefficients, where the current pixel is indicated by an asterisk. Errordiffusion halftoning is performed by repeatedly processing a currentpixel, and advancing the current pixel, pixel by pixel, scanline byscanline. Current pixel error is distributed to neighbouring unprocessedpixels shown in the diagram according to the fractions shown. That is,{fraction (129/256)} of a current pixel error is distributed to the nextpixel on the same scanline; and the remainder of the current pixel erroris distributed to 3 pixels on the following scanline. Note that the sumof the fractions adds to 1.

[0093] For each current pixel, a combined pixel input value isdetermined as the sum of the input image value, plus the errordistributed to the current pixel. The combined pixel input value is usedto index an error diffusion table 1514 in FIG. 17, or 1602 in FIG. 18,so as to determine (i) halftoned output eg. 1502, 1504 for the currentpixel, and (ii) error values eg. 1506, . . . , 1512, to be distributedfrom the current pixel to neighbouring unprocessed pixels

[0094]FIGS. 17 and 18 show error diffusion tables 1514, 1602 for 3 levelhalftone output suitable for use in printing super-intensity ink output.In the error diffusion tables 1602, 1514, errors to be distributed areshown as integer values equal to 256 times the fractional error value tobe distributed. The tables have been prepared using the error diffusioncoefficients of FIG. 16 and an algorithm described below. The algorithmcan be used to prepare a table based on a chosen set of image values(eg. “0”, “128”, and “255”, see FIG. 14) corresponding to halftoneoutput values (eg. 0, 1, 2, see FIG. 14), thereby tuning the halftoningtransfer function as previously described in relation to FIG. 14.

[0095] The error diffusion table of FIG. 17 has been prepared usingimage values of 0, 128, 255 corresponding to the 3 halftone outputvalues of 0, 1 and 2 (representing, respectively, no dots, 1 dot, andtwo dots per output pixel position). This is equivalent to thearrangement depicted in graph 1424 in FIG. 14. In contrast, the errordiffusion table of FIG. 18 has been prepared using image values of 0,128, 191 corresponding to the 3 halftone output values, this beingequivalent to the arrangement depicted in the graph 1426 in FIG. 14.

[0096] Because the image value corresponding to the maximum halftoneoutput level is reduced for the table of FIG. 18 in relation to FIG. 17,use of the table of FIG. 18 results in greater use of the super outputintensity ink deposition.

[0097] In both tables 1514, 1602, the halftone output 1604 is one of 3values, each being a pattern of bit values for the 2 output bits: bit o0(the least significant bit eg. 1606) and bit o1 (eg. 1608). The halftoneoutput value 2 (bit o0=0; bit o1=1) corresponds to the super intensityink deposition per pixel. Those rows in the tables which are not shown,but are indicated using a row of ellipsis ( . . . ) for example row1610, can be inferred from the preceding row and succeeding row of thetable. Both tables include a column 1612 for “combined pixel input”,which is shown as a sum of (i) an image value, and (ii) an error valuederived from the total error distributed to the current pixel. The tableentries with table index value in the range 320 to 448 (ie. 1516, 1614)in the tables of FIGS. 17 and 18 are not used. These unused tableentries are shown blank in FIGS. 17 and 18 and may be set to zero. Thetable entries e0, e1, e2, e3 (“errors distributed to neighbouringpixels” 1616-1618) with table index value in the range 255 to 319 in thetable of FIG. 18 (ie. 1620) have been determined using clamped pixelerror values. Error clamping is described below, as is an algorithm forpreparation of an error diffusion table based on a chosen set of imagevalues corresponding to the halftoned output levels,

[0098] Step 1.

[0099] For each image value, v, map the image value to the closest imagevalue corresponding to a halftone output level, out[v]. If there is ahalftone output image value above and below the image value which areequally close, choose the lower halftone output image value. From v andout[v] determine the error between them: err[v]=v−out[v]. Determine theminimum and maximum of these error values:

err_min=min_(vε0 225) err[v]

[0100] and

err_max=max_(vε0 . . . 255) err[v].

[0101] Step 2.

[0102] For each error augmented image value, v_aug, equal to an imagevalue plus an error in the range err_min to err_max, such that the erroraugmented image value is outside the range of image values, map thevalue to the closest image value corresponding to a halftone outputlevel, v_out[v_aug]. Again, if there is a halftone output image valueabove and below the error augmented image value which are equally close,choose the lower halftone output image value. From v_aug and out[v_aug]determine the error between them: err[v_aug]=v_aug−out[v_aug]. Note thatwhen the least image value corresponding to a halftone output level isgreater than 0, or when the greatest image value corresponding to ahalftone output level is less than 255, (as is the case for the table inFIG. 18) err[v_aug] may be outside the range err_min to err_max. Fromerr[v_aug], err_min and err_max, determine a clamped error valuerepresenting the difference between v_aug and out[v_aug] but which is inthe range err_min to err_max.

err_clamped[v_aug]=max(err_min, min(err_max, err[v_aug])).

[0103] Note that clamping error values ensures that during errordiffusion processing, the error cannot build up without bound.

[0104] Step 3.

[0105] Fill in the error diffusion table as follows. Firstly zero allentries in the table. For image values, v, the halftone output bitentries are determined as the halftone output level corresponding to theimage value, out[v] and each of the errors distributed to neighbouringpixels is determined by multiplying err[v] by the appropriate errordistribution coefficient, eg. FIG. 16. For error augmented image values,v_hd —aug, outside the range of image values, the halftone output bitentries are determined as the halftone output level corresponding to theerror augmented image value, out[v_aug] and each of the errorsdistributed to neighbouring pixels is determined by multiplyingerr_clamped[v_aug] by the appropriate error distribution coefficient,eg. FIG. 16.

[0106] Up to this point, the description has considered the case of asingle blocked nozzle, having fully functional nozzles adjacent thereto.Furthermore, the defective nozzle has thus far been considered to becompletely blocked, and the case of a partially functioning nozzle hasnot been addressed. Where there are two adjacent defective nozzles,compensation by means of either restricted, or full image valueredistribution, as described by the previous C code, can result in theresidual image value of the first defective nozzle being non-zero, evenafter compensation. This is because the image value of the firstdefective nozzle which remains after redistribution to the precedingnozzle cannot be redistributed to the succeeding second defectivenozzle. In order to ensure that the image redistribution method providesa consistent result, the residual image value of a defective nozzle canbe set to zero after defective nozzle compensation has been performed.This is particularly relevant when the defective nozzle is not, in fact,blocked but rather partially functional, producing a random type ofoutput. Thus for example, such a defective nozzle can sometimes eject arelatively large ink droplet, and at other times can eject a relativelysmall ink droplet, both for the same input firing value. Setting theimage value of such a defective nozzle to zero after image valueredistribution has been performed ensures that any possible residualvalue remaining associated with the defective nozzle does not cause thenozzle to fire.

[0107] The description thus far has considered image valueredistribution to immediately neighbouring nozzles, this being the mostsimple example to describe. In this type of compensation, surplus ink isdeposited by the immediately neighbouring nozzles in order to compensatefor the deficit, or lack of ink deposited by the defective nozzle. Amore complex redistribution scheme involving, for example, first andsecond neighbouring pixels of a blocked nozzle can also be considered,where the expected benefits of such a scheme can justify the increase incomplexity, Alternatively, use of head shading data prepared fromprintout generated by using defective nozzle compensation by image valueredistribution to first neighbour nozzles can provide some compensationfor the surplus local average ink deposition introduced for secondneighbour nozzles.

[0108] Defective nozzle compensation by image value redistribution hasthus far been described on the assumption that defective nozzle dataconsists of a 1 bit value for each nozzle, indicating a fullyoperational state, or alternatively, a fully defective state. Unwantedprint artefacts can, in some cases, be further reduced by extending thedefective nozzle data description to more than a binary description. Insuch an event, the degree to which image value is redistributed awayfrom a defective nozzle can be controlled according to the finergranularity of the provided defective nozzle data.

[0109] The aforementioned description has been directed to defectivenozzle compensation by image value redistribution in respect of fixedprint head systems. Clearly, this can also be applied to shuttleheadprint systems and the like. In respect of shuttlehead print systems,defective nozzle compensation by image value redistribution isparticularly effective when the shuttle printer is performing “one pass”printing, since in that case dots of a colour component of a scanlineare printed by only a single nozzle.

[0110] The method of compensating for a defective printer ink nozzle canbe practiced using a conventional general-purpose computer system 1700,such as that shown in FIG. 19 wherein the processes of, for example,FIGS. 4, 10 and 13 can be implemented as software executing within thecomputer system 1700. In particular, the steps of the method ofcompensating for a defective printer ink nozzle are effected byinstructions in the software that are carried out by the computer. Thesoftware can be divided into two separate parts, one part for carryingout the compensating for a defective printer ink nozzle, and anotherpart to manage the user interface between the latter and the user. Thesoftware can be stored in a computer readable medium, including thestorage devices described below, for example. The software is loadedinto the computer from the computer readable medium, and then executedby the computer. A computer readable medium having such software orcomputer program recorded on it is a computer program product. The useof the computer program product in the computer effects an advantageousapparatus for compensating for defective print nozzles in accordancewith the arrangements of the invention.

[0111] The computer system 1700 comprises a computer module 1701, inputdevices such as a keyboard 1702 and mouse 1703, output devices includinga printer 1715 and a display device 1714. A Modulator-Demodulator(Modem) transceiver device 1716 is used by the computer module 1701 forcommunicating to and from a communications network 1720, for exampleconnectable via a telephone line 1721 or other functional medium. Themodem 1716 can be used to obtain access to the Internet, and othernetwork systems, such as a Local Area Network (LAN) or a Wide AreaNetwork (WAN).

[0112] The computer module 1701 typically includes at least oneprocessor unit 1705, a memory unit 1706, for example formed fromsemiconductor random access memory (RAM) and read only memory (ROM),input/output (I/O) interfaces including a video interface 1707, and anI/O interface 1713 for the keyboard 1702 and mouse 1703 and optionally ajoystick (not illustrated), and an interface 1708 for the modem 1716. Astorage device 1709 is provided and typically includes a hard disk drive1710 and a floppy disk drive 1711. A magnetic tape drive (notillustrated) can also be used. A CD-ROM drive 1712 is typically providedas a non-volatile source of data. The components 1705 to 1713 of thecomputer module 1701, typically communicate via an interconnected bus1704 and in a manner which results in a conventional mode of operationof the computer system 1700 known to those in the relevant art. Examplesof computers on which the embodiments can be practised include IBM-PC'sand compatibles, Sun Sparcstations or alike computer systems evolvedtherefrom.

[0113] Typically, the program of the preferred embodiment is resident onthe hard disk drive 1710 and read and controlled in its execution by theprocessor 1705. Intermediate storage of the program and any data fetchedfrom the network 1720 can be accomplished using the semiconductor memory1706, possibly in concert with the hard disk drive 1710. In someinstances, the program can be supplied to the user encoded on a CD-ROMor floppy disk and read via the corresponding drive 1712 or 1711, oralternatively can be read by the user from the network 1720 via themodem device 1716. Still further, the software can also be loaded intothe computer system 1700 from other computer readable medium includingmagnetic tape, a ROM or integrated circuit, a magneto-optical disk, aradio or infra-red transmission channel between the computer module 1701and another device, a computer readable card such as a PCMCIA card, andthe Internet and Intranets including email transmissions and informationrecorded on websites and the like. The foregoing is merely exemplary ofrelevant computer readable mediums. Other computer readable mediums canbe practiced without departing from the scope and spirit of theinvention.

[0114]FIG. 20 depicts a block diagram representation 2000 of anapparatus for redistribution of image values from a defective nozzle toneighbouring non-defective nozzles. The apparatus 2000 has a number ofimage forming elements 2002, in respect of which, relative desirabilitydata is stored, as depicted by a dashed line 2004, in a memory 2006.Input image values, depicted by an arrow 2012 are input into a processor2008, as well as desirability data from the memory 2006. The processor2008 consequently outputs biased image recording signal data, asdepicted by an arrow 2010, to the forming elements 2002, which therebyform the desired image.

[0115] The method of compensating for a defective printer ink nozzle canalternatively be implemented in dedicated hardware such as one or moreintegrated circuits performing the functions or sub functions ofcompensating for a defective printer ink nozzle. Such dedicated hardwarecan include graphic processors, digital signal processors, or one ormore microprocessors and associated memories.

[0116] Industrial Applicability

[0117] It is apparent from the above, that the embodiments of theinvention are applicable to the digital image printing industry. Imagevalue redistribution provides an effective and computationally simplemethod of compensating for defective nozzles. Furthermore, this can becombined with existing unevenness correction methods. Restricted imagevalue redistribution can be combined with cross-colour compensation forimproved defective nozzle compensation. Full image value redistributionand super output intensity printing can be combined with checkerboardquantisation for ease of implementation, and can be used with modifiederror diffusion tables, in order to calibrate how often the super outputintensity ink deposition per pixel is used.

[0118] The foregoing describes only some embodiments of the presentinvention, and modifications and/or changes can be made thereto withoutdeparting from the scope and spirit of the invention, the embodimentsbeing illustrative and not restrictive.

1. A method, in printing an image, of compensating for one or moredefective printer nozzles in a plurality of printer nozzles, said methodcomprising the steps of. biasing, for each first image value associatedwith a first nozzle, at least one second image value associated withanother nozzle, said biasing being dependent upon said first image valueand a term for said first nozzle; and printing the image in accordancewith the biased image values, said biasing reducing print artefactsotherwise caused by the one or more defective nozzles.
 2. A methodaccording to claim 1, whereby the term for said first nozzle provides ameasure of one of effectiveness and defectiveness of said first nozzle.3. A method according to claim 1, whereby said biasing comprises thesub-step of: redistributing one of part of said first image value andall of said first image value to one or more image values associatedwith immediately neighbouring nozzles of a same colour.
 4. A methodaccording to claim 3, whereby an extent of image value redistribution isdependent upon an allowed operating range of the one or more imagevalues associated with said immediately neighbouring nozzles.
 5. Amethod according to claim 4, whereby said allowed operating range ofsaid image values is between 0% and 100%, wherein 100% represents amaximum intensity for unbiased image values.
 6. A method according toclaim 4, whereby said allowed operating range of said image values isbetween 0% and 200%, wherein 100% represents a maximum intensity forunbiased image values, and 200% represents a super-intensity for biasedimage values.
 7. A method according to claim 1, whereby said biasingcomprises the sub-steps of: increasing an image value associated with acorresponding nozzle of another colour.
 8. A method according to claim1, whereby said biasing comprises the sub-steps of: redistributing oneof part of said first image value and all of said first image valueassociated with said first nozzle to one or more image values associatedwith immediately neighbouring nozzles of a same colour; and increasingan image value associated with a corresponding nozzle of another colour,said increase being dependent upon a residual image value of said firstnozzle after said redistribution step.
 9. A method according to claim 6comprising, prior to printing the image, the sub-step of: mapping thebiased image values from a biased image value range of 0% to 200%, to arange of 0% to 100%.
 10. A method according to claim 9, whereby saidmapping uses checkerboard quantisation, said method comprising the stepsof: dividing said biased image values by 2; and alternately roundingsuccessive divided image values up, and down.
 11. A method according toclaim 6, comprising, prior to printing the image, the sub-step of:halftoning the biased image values.
 12. A method according to claim 11,whereby, in a multi-level halftoning, process, a relationship between aninput image value and a corresponding average halftone output value isadjusted in order to tune a utilisation of super-intensity printing. 13.A method of printing a multi-level halftoned image comprising the stepsof: adjusting a relationship between input image values andcorresponding average halftone output values using an error diffusiontable.
 14. An image recording apparatus comprising: (a) a plurality offorming elements for forming an image according to input image formingsignals; (b) memory means for storing data for said forming elementsindicating the relative desirability of utilising said forming elementsfor forming an image; (c) image processing means for computing imagerecording signals using said input image forming signals and said datastored in said memory means where the use of a forming element is biasedusing the relative desirability data of other forming elements.
 15. Animage recording apparatus comprising: (a) a plurality of formingelements for forming an image using image recording signals, said imageaccording with a corresponding plurality of input image forming signals;(b) memory means for storing data for said forming elements indicatingthe relative desirability of utilising said forming elements for formingthe image; and (c) image processing means for computing said imagerecording signals using said input image forming signals and said datastored in said memory means, wherein the use of a particular formingelement is thereby biased dependent upon the relative desirability dataof other forming elements, the corresponding input image forming signalfor the particular forming element, and a term for the particularforming element.
 16. An image recording apparatus comprising: (a) aplurality of forming elements for forming an image according to inputimage forming signals; (b) memory means for storing data for saidforming elements indicating the relative desirability of utilising saidforming elements for forming an image; (c) image signal modificationmeans for redistributing values of said input image forming signal basedon said data stored in said memory means so as to bias the use of saidforming elements.
 17. An image recording apparatus comprising: (a) aplurality of forming elements for forming an image according to inputimage forming signals; (b) memory means for storing data for saidforming elements indicating the relative desirability of utilising saidforming elements for forming an image; (c) image signal modificationmeans for redistributing values of said input image forming signalsbased on said data stored in said memory means so as to bias the use ofsaid forming elements, wherein the use of a particular one of saidforming elements is thereby biased dependent upon the relativedesirability data of other forming elements, a corresponding input imageforming signal for the particular forming element, and a term for theparticular forming element.
 18. An image recording apparatus accordingto claim 16 where: said image signal modification means forredistributing values of said input image forming signals does notextend the range of said values.
 19. An image recording apparatusaccording to claim 14 where: said apparatus is a colour image recordingapparatus, said plurality of forming elements including plural groups offorming elements respectively corresponding to colour components.
 20. Animage recording apparatus according to claim 19 where: said imageprocessing means includes means for modifying the input image formingsignals relating to a colour component based on said input image formingsignals and based on said data indicating the relative desirability ofutilising said forming elements relating to other colour components. 21.An image recording apparatus according to claim 16 where: said apparatusis a colour image recording apparatus, said plurality of formingelements including plural groups of forming elements respectivelycorresponding to colour components.
 22. An image recording apparatusaccording to claim 21 further comprising: image processing means formodifying said redistributed input image forming signals relating to acolour component based on said redistributed input image forming signalsand based on said data indicating the relative desirability of utilisingsaid forming elements relating to other colour components.
 23. An imagerecording apparatus according to claim 16 where: said forming elementsare capable of recording a “super” density being greater than anydensity recorded by said forming elements when no image forming signalvalues are redistributed by said image signal modification means; andsaid image signal modification means is capable of biasing the use ofsaid forming elements to record said super density.
 24. An imagerecording apparatus according to claim 23 where: redistribution ofvalues of said input image forming signals is capable of extending therange of said values.
 25. An image recording apparatus according toclaim 24 further comprising: image processing means for re-mapping saidredistributed image forming signals so that the range of said values isrestored to the range existing prior to said redistribution.
 26. Animage recording apparatus according to claim 24 where: said imageprocessing means map redistributed image forming signals to the rangeexisting prior to said redistribution by maintaining distinct localaverage image values for image regions with differing constant inputimage signal value.
 27. An image recording apparatus according to claim26 where: said image processing means map redistributed image formingsignals to the range existing prior to said redistribution bysubstantially dividing image values by 2 and alternately rounding up androunding down.
 28. An image recording apparatus according to any ofclaims 24 to 27 further comprising: halftoning means which generaterecording element signals so that the frequency of occurrence of superdensity recording by recording elements is adjusted according tohalftoning parameters.
 29. An image recording apparatus according toclaim 28 where: said halftoning means generate recording element signalsby error diffusion processing such that the frequency of occurrence ofsuper density recording by recording elements is adjusted according tovalues in an error diffusion table.
 30. An image recording apparatusaccording to claims 14 to 29 further comprising: image signal forcingmeans whereby image signals corresponding to selected forming elementsare set to prevent recording; said selected forming elements beingdetermined by said data indicating the relative desirability ofutilising forming elements.
 31. An image recording apparatus accordingto claims 16 to 30 further comprising: (a) memory means for said formingelements based on non-uniformity of the density of a recorded testimage; and (b) correction means for correcting said redistributed inputimage forming signals based on said data stored in said memory means.32. An image recording apparatus according to claims 14 to 31 wherein:each of said forming elements is a forming element for ejecting a liquiddrop by film-boiling due to head energy.